1. Field of Invention
The present invention relates to a spark plug used as an ignition device for internal combustion engines. The present invention particularly relates to a spark plug for use with high-power and high-performance internal combustion engines such as rotary engines and reciprocal engines of high compression ratio.
2. Description of the Related Art
In high-power and high-performance internal combustion engines, the standard spark plug using parallel electrodes can not be used due to not only the mechanical strength problems such as the low heat resistance and breaking of the ground electrode but also the problem of carbon fouling during vehicle running under low lead. Instead, there have been used spark plugs of a semi-surface discharge type or an intermittent semi-surface discharge type that have a plurality of ground electrodes provided to face the peripheral side surface of the central electrode. A problem with these spark plugs of a semi-surface discharge type is how to improve the spark resistance and reduce the consumption of the central electrode. According to Unexamined Japanese Patent Publication (kokai) No. 6-176849, there is provided a spark plug in which an anti-spark consumption member typically made of a platinum alloy is put around the central electrode in an area near the end face of the porcelain insulator in such a way that about one half of the anti-spark consumption member is buried in the porcelain insulator. This is effective in preventing the spark consumption of the central electrode. If the surface of the porcelain insulator is fouled with carbon, surface discharge is caused to achieve spark cleaning. The spark plug had adequate firing performance and its operating life was satisfactory at the time it was invented.
As it turned out, however, this conventional spark plug did not have a sufficient life to meet the current requirement. Heretofore, high-performance spark plugs have not been required to have a very long life and they have been held satisfactory if they can withstand running for 50,000 to 60,000 km. However, in recent years, even the high-performance spark plugs are required to have a sufficient life to withstand running for 100,000 to 120,000 km. This requirement cannot be met by the spark plug described in Unexamined Published Japanese Patent Publication (kokai) No. 6-176849 since the surface of the porcelain insulator is grooved by spark discharge. This problem called xe2x80x9cchannelingxe2x80x9d has been found to occur for the following reasons.
In the spark plug as described in Unexamined Japanese Patent Publication (kokai) No. 6-176849, an anti-spark consumption member typically made of a platinum alloy is put around the central electrode in an area near the end face of the porcelain insulator in such a way it is partly buried in the porcelain insulator. In the spark plug, if it is new with the porcelain insulator being not fouled with carbon, about 70% of spark jumps occur between the top of the central electrode and the side ground electrode. The remaining 30% of spark jumps occur as a surface discharge that creeps on the end face of the porcelain insulator. of course, if the surface of the porcelain insulator is fouled with carbon, spark jumps exclusively occur as a surface discharge to cause the spark cleaning of the porcelain insulator.
However, after the use equivalent to running for several tens of thousand kilometers, the distal end portion of the central electrode that is not encircled with the anti-spark consumption member is consumed by spark discharge. This increases the distance between the distal end portion of the central electrode and the side ground electrode and, hence, the discharge distance is increased to make it difficult to achieve spark jumps. As a result of the spark consumption of the distal end portion of the central electrode, the nearby electrical field would have been relaxed. Consequently, the primary discharge that occurs. in the spark plug is the surface discharge that is caused by the jumping of electricity between the neighborhood of the base of the central electrode which is encircled with the anti-spark consumption member and the side ground electrode. Thus, after running for several tens of thousand kilometers, the discharge that primarily takes place in the spark plug is the surface discharge that creeps on the end face of the porcelain insulator and the progress of xe2x80x9cchannelingxe2x80x9d is accelerated. If xe2x80x9cchannelingxe2x80x9d progresses, the mechanical strength such as heat resistance of the spark plug is impaired or its reliability is lowered, which eventually leads to a shorter operating life of the spark plug.
It is an object of the present invention to provide a spark plug for use with high-power, high-performance internal combustion engines that has high resistance to not only fouling but also channeling to be capable of operating for a prolonged life.
A spark plug according to the present invention comprises: a porcelain insulator having a central through-hole; a central electrode held in the central through-hole, the central electrode having a distal end provided with an anti-spark consumption member; a metal shell holding the porcelain insulator; and a plurality of ground electrodes having electrical continuity to the metal shell, the plurality of ground electrodes forming a spark discharge gap from the distal end portion of the central electrode The shortest distance from the end face of each ground electrode to the porcelain insulator is smaller than the spark discharge gap. The said distal end of the central electrode projects from an end face of the porcelain insulator. The central electrode comprises a central electrode matrix in a plane coextensive with the end face of the porcelain insulator.